The invention is used for assemblies which are intended for pressurized water reactors (PWR), in contrast to boiling water reactors (BWR), and whose nuclear fuel is uranium which is enriched in isotope 235.
These assemblies are generally designated UO2 assemblies, with reference to the nature of their fuel.
This term UO2 is used in contrast to assemblies having fuel with mixed uranium and plutonium oxide which are generally designated MOx assemblies.
MOx assemblies of this type allow the plutonium which originates from the reprocessing of UO2 assemblies to be reused. Document FR-2 693 023 describes a MOx assembly of this type. UO2 assemblies and MOx assemblies have different neutron behaviour. In order to nonetheless allow MOx and UO2 assemblies to be simultaneously loaded in the same reactor, this document has proposed that rods with different plutonium contents be used in the MOx assemblies. “Zoned” MOx assemblies are therefore referred to since these assemblies comprise zones in which the rods have different plutonium contents.
As has already been indicated above, the present invention does not relate to MOx assemblies but instead applies to UO2 assemblies which do not have zone arrangements of this type, the enrichment in isotope 235 being uniform in this case. It is true that EP-799 484, for example, discloses UO2 assemblies of which a few isolated rods are contaminated with gadolinium and have an enrichment in uranium 235 which is less than that of the adjacent rods. However, these are not zoned assemblies in the strict sense.
A UO2 assembly comprises a skeleton for retaining the fuel rods in the nodes of a regular network which generally has a square base. The skeleton comprises a lower end, an upper end, guiding tubes which connect the two ends and grids for retaining the fuel rods.
Within the core of a pressurized water nuclear reactor, the UO2 assemblies are arranged beside each other with a slight lateral spacing in the order of 2 mm. This spacing in particular allows the assemblies to be raised and lowered during operations for loading and unloading the core.
The cooling and moderation water flows in the gaps which result from this spacing and forms layers of water at that location.
The height of assemblies of this type is great and can be up to three or four metres. Owing to production tolerances, the actual thickness of the layers of water could, at least locally, be different from the nominal thickness of 2 mm.
Furthermore, assemblies which are placed in a reactor could theoretically become deformed owing to irradiation resulting in, for example, C, S or W-like shapes.
Deformations of this type would present a number of problems. During operation, they would make it more difficult to insert the control and stop rod clusters of the nuclear reactor in the guiding tubes.
During handling, these deformations would increase the risks of the assemblies becoming hooked together, for example, during operations for loading the core of the reactor.
The actual behaviour of the UO2 assemblies could thus be different from that which is desired, at least in mechanical terms.